The present invention relates to aircraft head-up display (HUD) systems and, in particular, to HUD display strategies and symbologies to aid a pilot in detecting and recovering from unusual aircraft attitudes (climb/dive pitch and or roll) during flight.
HUD systems are now widely used in both military and commercial aircraft to provide pilots with essential information superimposed onto their forward field of view through the aircraft windshield. The information displayed is typically data and/or symbolic images indicative of flight conditions, such as aircraft altitude or airspeed, navigation information, or guidance information.
FIG. 1 generally illustrates a side view of the optics geometry of a head up display system in the cockpit 10 of an aircraft 11 having a windshield 12. A pilot is shown at 13 with an eye shown at an eye reference point 14 within an eyebox (exit pupil) indicated by dashed lines 15. The eye reference point 14 is a geometrical point in space. FIG. 2 illustrates a field of view from the pilot to the combiner 20.
Guidance information displayed on a HUD combiner for observation by a pilot generally includes symbology that represents position and attitude guidance for the aircraft during flight. One example is a flare anticipation cue implemented in a Flight Dynamics head-up guidance system that is certified by the Federal Aviation Administration for use on Boeing 737-300 aircraft. The flare anticipation cue alerts the pilot several seconds before the aircraft reaches a flare initiation height and indicates to the pilot the pitch up rate required at the initial part of the flare.
Another application of head-up displays is guidance information that can be displayed to a pilot during low visibility ground weather conditions to assist the pilot after touchdown in safely taxiing the aircraft to the airport terminal. A pending patent application, also owned by the assignee of the present application, describes methods and symbologies for taxi mode of operation, including the production of a turn anticipation cue that, together with a turn direction arrow, is displayed as a symbol on a HUD system combiner as part of aircraft rollout on an active runway and taxi operations to and from an airport terminal. The turn anticipate cue alerts the pilot that a turn from the current path of the aircraft is approaching within a predetermined time or distance. HUD displays thus can be used in various modes of operation, including a basic mode, en route mode, approach/landing mode and a taxiing mode as noted.
One very important yet challenging task is to design a HUD display to assist the pilot in recognizing and recovering from an unusual attitude (UA) of the aircraft. One important aspect of the problem is to enable the pilot to quickly and accurately identify which way is up (away from the center of the earth). In a heads-down display (HDD), color is used to advantage in this regard. An attitude display or attitude indicator ball in the HDD displays a straight line representing the horizon; solid brown color below the horizon (i.e. toward the ground), and a solid blue color field above the horizon to indicate the direction of the sky. That combination of indicia makes it easy for the pilot to quickly identify which way is up. Most head-up displays, however, are limited to simple monochromatic symbologies: Solid fields, shading, cross-hatching and the like generally are not used because they occlude the view through the windshield outside the aircraft. Consequently, attitude indicators like the HDD cannot be used for HUD.
Another challenge in addressing unusual attitudes in HUD arises because HUD""s provide a very limited field of view (FOV). For example, a typical HUD today provides a 30 degree by 24 degree FOV (The dimensional units most often used to describe symbol size and position are miliradians (mrad) of angular arc. Either way, a specified viewing distance is required to convert to equivalent linear dimensions of the display). Given the relatively limited FOV available in the HUD, display of a wide range of conformal climb/dive pitch angles is not possible. One means to increase the range of angles displayed is through pitch compression. However, compression of the pitch ladder is undesirable, at least in some circumstances, as it may to lead to disorientation. Put another way, a xe2x80x9cconformalxe2x80x9d or one-to-one display on the HUD consistently tracks the xe2x80x9creal worldxe2x80x9d that the pilot is viewing through the windshield. Compression seems necessary in order to accurately display the more extreme excursions in pitch and roll that occur during unusual UA""s, yet conformal orientation cues may be superior in just those situations. There is some controversy as to the most appropriate approach. If full-time scaling is not selected, this issue is further complicated by consideration of switching between compressed and noncompressed display modes; for example, should a compression mode switch take place gradually or in a discrete step. See Newman at page 87.
FIGS. 3 and 4 illustrate a typical prior art HUD display. The features and symbols employed in this type of display shown in FIG. 3 and 4 include the following: to the left side of the display is an air speed indicator 302, in this case a xe2x80x9ctapexe2x80x9d display combining both analog and digital features. A digital ground speed indicator 304 also is provided in the lower left quadrant of the display. Other known air speed indicators include digital and counter-pointer symbols. To the right side of the display is an altitude indicator 310. In this case, an altitude xe2x80x9ctapexe2x80x9d is shown. A digital vertical speed indicator 306 also is provided generally in the lower right hand quadrant of the display. Again, other types of altitude displays are known in the art. Referring to FIG. 4, the display 300 includes a horizon indicator 312, in this case a straight line segment with no break. A conventional pitch ladder 314 is provided for indicating climb/dive attitude in conjunction with the horizon 312.
The pitch ladder 314 in FIG. 4 includes a series of steps, spaced apart typically in 5 degree pitch increments. There, the steps are labeled 5, 10, (0 is the horizon line 312), and finally minus 5 degrees. Each pitch step indicator comprises a pair of line segments with a space in the middle, symmetrically arranged about the water line indicator 316, and having perpendicular ends on the outboard side of each line segment pointing toward.the horizon; for example, see reference number 315. The water line symbol 316, also called a bore sight, indicates an axis parallel to the fuselage of the plane and generally remains in a fixed position near the center of the FOV.
An airplane: symbol 320 in FIG. 4 indicates the current flight path of the aircraft. In the display of FIG. 4, the aircraft is in what would generally be considered an unusual attitude. The display indicates a pitch of about 2 degrees and a roll, angle of about 140 degrees. Here, an xe2x80x9cAugie arrowxe2x80x9d 322 is displayed to point toward the horizon. The Augie arrow has been suggested for use during unusual attitudes as an orientation cue, but we consider it too small and too subtle and not compelling enough to be useful in identifying and recovering from a UA.
FIG. 5 is another example of a prior art HUD display, including most of the features and symbols already described with reference to FIG. 4. In FIG. 5, the horizon 330 is shown as a dashed line, known as a ghost horizon. In this case, the pitch ladder provides the usual 5 degree steps; and it can be observed that 0 degrees pitch is just about where the horizon 330 is shown in the drawing. As the pitch ladder moves further down in the display (presumably as the aircraft assumes a greater pitch), the horizon would no longer be visible on the display (assuming no automatic compression of the ladder). The ghost horizon is dashed to indicate that it is no longer a conformal representation of the horizon relative to the pitch ladder. However, albeit dashed, the horizon symbol 330 remains visible on the display at all times, thereby providing at least a minimal orientation cue as to which direction is up or down.
One aspect of the present invention provides for a dramatic change in the HUD display appearance when a UA is encountered during flight. The UA is defined as a climb/dive pitch angle and/or a roll angle that exceeds pre-defined limits. These can be individually programmed and will depend upon a particular aircraft and other variables. The HUD system is programmed to monitor pitch and roll, and switch into a UA display mode when a UA is encountered. The switch to UA display mode should be discrete rather than gradual, and it may be punctuated by flashing the entire display screen several times within the span of about one second. The UA mode of display is characterized by decluttering the display so as to remove most of the non-essential symbols that appear during other modes of operation. What remains on the UA display mode is essentially three components: an air speed indicator, an altitude indicator and an attitude indicator. In a presently preferred embodiment, the air speed indicator is shown on the left side of the display in the form of a conventional air speed tape. The right side of the display features an altitude display tape. The center of the display in UA mode consists of a large, unitary attitude display element, in the form of a single closed shape such as a circle or rectangle.
Within the attitude display, absence of clutter remains important, but the following symbols generally should be included: a horizon, a center line perpendicular to the horizon, at least a bilaterally symmetrical pair of sectional lines on either side of the center line and converging toward the horizon; a conformal pitch ladder aligned with the center line (i.e. generally perpendicular to the horizon); a water line indicator at the center of the attitude display; and a roll scale and roll indicator extending along at least a portion of the periphery of the attitude display component.
In one embodiment, the attitude display is circular, referred to as a UA ball display (UABD). Regardless of the selected shape of the attitude display, the horizon appears within the display at all time, even if truncating the pitch ladder becomes necessary in order to preserve the horizon display.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.